Signal-transmitting system including an optical limb with automatic amplitude limitation

Abstract

A fiber-optical communication path between a transmitter and a receiver of electrical message signals forms a main channel and a shunt branch terminating at respective photodiodes from which incoming signals can be alternatively fed to an output terminal by way of an adjustable attenuator that is periodically reset, with the aid of a digitally operating automatic-gain-control loop in response to a pilot signal arriving over the same path. A code converter in that loop supplies binary words, representative of signal amplitude, to a comparator which determines whether the amplitude of a signal in the output of the main channel exceeds a given threshold and, in that case, causes a switchover to the shunt branch whose optical attenuation is substantially greater. The periodic emission of a pilot signal in lieu of a message signal is controlled by a setting instruction which, aside from enabling the resetting of the attenuator according to the instantaneous contents of the code converter, is sent to the transmitting end via a further fiber-optical channel in order to command a changeover from one signal source to another.

I claim: 
     
       1. A system for conveying message signals from a transmitter to a receiver by way of a communication path, comprising: fiber-optical transmission means forming part of said communication path, said transmission means including a main channel extending from an input end in said transmitter to a first output end and a branch channel extending between said input end and a second output end, said branch channel having a higher optical attenuation than said main channel;   an emitter of optical signals confronting said input end;   a first and a second photoelectrical transducer respectively confronting said first and second output ends for translating incoming optical signals into electrical signals;   a load circuit in said receiver;   changeover means with a normal position and an off-normal position for respectively connecting said first and said second transducer to said load circuit; and   amplitude-sensing means connected to said load circuit and effective in said normal position of said changeover means for switching same to said off-normal position upon detecting a signal amplitude exceeding a predetermined reference level.   
     
     
       2. A system as defined in claim 1 wherein said load circuit includes signal-amplifying means provided with an automatic-gain-control loop, said amplitude-sensing means being controlled by said arrangement. 
     
     
       3. A system as defined in claim 2 wherein said loop includes an analog-to-digital converter receiving said electrical signals from said signal-amplifying means, a transcoder supplied with binary words from said converter, and an attenuator downstream of said signal-amplifying means with a control input connected to said transcoder for adjustment of the attenuation thereof in response to said binary words, said amplitude-sensing means comprising a comparator connected to said transcoder and to a supply of a predetermined reference word representing a limiting amplitude. 
     
     
       4. A system as defined in claim 3 wherein said transmitter includes switch means for alternately connecting said emitter of optical signals to a source of message signals and to a source of pilot signals in response to an instruction signal from said receiver, further comprising control means in said receiver responsive to said instruction signal for enabling an adjustment of said attenuator by a binary word only upon reception of a pilot signal over said communication path. 
     
     
       5. A system as defined in claim 4, further comprising another fiber-optical channel extending from said receiver to said transmitter for conveying said instruction signal to said switch means. 
     
     
       6. A system as defined in claim 1, 2, 3, 4 or 5 wherein said main channel and said branch channel comprise respective optical fibers fused to each other at an intermediate point of said main channel.